Traditionally telephone services have been provided by telephone switching systems each being linked to a multitude of telephone station sets via telephone lines which carry direct operating current and alternating currents of voice band frequencies and lower. These telephone services are limited to those communication services providable within the voice bandwidth. Sometime ago, it became common practice to provide long distance digital trunks between various switching systems by means of pulse code modulated time division multiplexed (PCM TDM) carrier, for example T1 carrier which provides twenty-four digital signal channels each of sixty-four kilobits per second. This required multiplexing and demultiplexing, and encoding and decoding of each analog voice band signal for transmission and reception.
Recently PCM TDM telephone switching systems having been installed to the extent that about forty percent of the telephone switching facilities in North America are of the digital type. Typically in such systems each subscriber telephone line carries analog signals and is connected to the telephone switch by a line interface circuit. Each line interface circuit includes a CODEC which performs analog to digital signal conversion and digital to analog signal conversion. Each line interface circuit also includes signalling and supervision circuitry for detecting ON HOOK and OFF HOOK conditions. Some examples of such PCM TDM telephone systems ar those manufactured by the assignee and sold under the registered trademarks SL and DMS. These systems are representative of an evolution in telephony which has increased the quality and efficiency of telephone services. However, despite the digital capability of modern telephone exchanges, available telephone services generally continue to be limited to those providable within the voice band available on analog telephone lines.
Recently, digital telephone service for voice and data has been available in some private branch exchanges (PBX) digital systems, via proprietary digital telephone lines and interface circuits. One problem with proprietary digital telephone lines is that corresponding proprietary station or terminal apparatus are required in order to compatibly communicate via the switching system. In the realm of private branch exchanges (PBXs) and private networks, this restrictive requirement has proven to be inconvenient. However, in the realm of public telephone networks, the inability to communicate digitally via apparatus of different manufacture and hence different protocol requirements has rendered many potential advanced telephone services to be simply impractical.
This impracticality has long bedeviled the telephone industry. It was only in 1984 that The International Telegraph and Telephone Consultative Committee (CCITT) of the International Telecommunication Union established recommendations for a standard Integrated Services Digital Network (ISDN). This recommendation was published in Geneva Switzerland in 1985 with an identification number ISBN 92-61-02081-X.
ISDN is an all-digital network standardized concept intended to provide end-to-end digital connectivity to support a wide range of voice services and non-voice services, for example, data and video services. These recommendations are the basis for the ISDNs currently being planned for deployment. ISDN subscribers will have access to these services through a number of internationally standardized, multi-purpose user network interfaces. ISDNs are evolving from existing digital telecommunications networks, by progressively incorporating additional functions and network features so as to provide users with a standard integrated access to both existing and new services. Various manufacturers of fully digital telecommunications equipment are about to, or have, supplied equipment for ISDN field trials and are committed to build equipment for full scale deployment. This equipment is typically based upon functional integrations of existing digital circuit switches and packet switches.
FIG. 1 of the accompanying drawings illustrates one prior art example of a typical time division switching system which is readily adaptable to serving as an exchange termination (ET) in an integrated services digital network (ISDN) by means of a D channel handler. This switching system is described in U.S. Pat. No. 4,213,201.
Briefly, the system illustrated in FIG. 1 includes four principal areas, namely, a peripheral modules area 1, a network area 2, a central control complex area 3, and a maintenance and administration area 4. The switching network contained in the network area 2 is a so-called folded network. The switching network is duplicated for reliability as illustrated by identical networks labelled as "plane 0" and "plane 1". The peripheral modules area 1 contains three kinds of peripheral modules. For example, the line module serves local telephone lines carrying analog speech signals which are digitized and grouped into time division multiplex groups of thirty-two channels, of which thirty channels in each group are used for communication with the duplicated switching network planes via network links. The line module can be regarded as a stage of time division switching because it provides concentration in contrast to a trunk module which normally connects thirty trunks to thirty duplicated network link channels on a non-blocking basis. Both the line and trunk modules also provide conversion for analog and pulse code modulated signal formats in contrast to the digital carrier module which merely provides a reframing interface between digital carrier facilities and the switching network planes.
The central control area 3 of the switching system, like the network area 2, contains facilities all of which are duplicated for reliability in the system. As shown in FIG. 1, central message controllers (CMCs) are connected by control signal links to both of the network planes. Likewise two central processing units are each connected by parallel buses to both of the CMCs. Data stores 0 and 1 and program stores 0 and 1 are connected, as shown, to the central processing units. The CMCs are each connected a to network module controller (NMC), not shown, in the switching network planes 0 and 1 by the control signal links. Equipment of the maintenance and administration area 4 are interconnected with the equipment of the central control area 3 through the CMCs.
Call processing is hierarchically distributed between the central control (CC) area 3 and the peripheral module (PM) areas 1. For example, translation is done in the CC area 3, while digit collection and call supervision are handled in the PMs area 1.
In operation of the switching system in FIG. 1, the CPUs of the CC area 3 are responsive to signals received from at least one of the PMs to set up and tear down communications channels between various of the lines and trunks served by the line, trunk and carrier modules in the PMs 1. Signals representing requests for service, and subsequent call progress and control of the switching network, are passed through the network 2 on at least one of the two remaining channels of the thirty-two channels of each time division group. One example of an analog subscriber interface circuit which communicates signalling and supervision messages via a signalling channel is described by Harold Harris in an article titled "The Line Card" published by Bell-Northern Research Ltd. in TELESIS, the fourth issue of 1980. The CMCs function is to both assemble messages from the signalling channels for presentation to the central processing units and to distribute messages from the central processing units to the appropriate signalling channels.
The principles of the present invention as embodied in the ISDN D channel handler and its application, are useful in an adaptation of virtually any TDM PCM telephony switching system to ISDN, as well as being applicable to designs of future switching systems.
The CCITT recommendation for ISDNs defines several layers of standard protocols, which when adhered to, permit open digital communication between terminal and station apparatus of various equipment manufacture via circuit switched digital telephone systems. The ISDN basic interface protocol defines a signal format for a subscribed loop. The signal format includes two sixty-four kilobits per second channels, termed B channels, and a sixteen kilobits per second channel, termed a D channel. The B channels are used for encoded voice and for data and are normally intended to be circuit switched in an exchange termination (ET), for example an associated TDM switching facility. The D channel is available for at least two uses, one being an exchange of supervisory and signalling information between a subscriber terminal or telephone station set and the ET, and the other being communication of packet data via the ET and a packet communication network, which is linked to the ET.
In the ET, ISDN formatted signals received from an ISDN subscriber line are separated into the B and D channel components. One or both of the B channels may be circuit switched in the ET. The D channel may be communicated to a packet network or alternately any supervisory and signalling information in the D channel is ultimately communicated to a central controller of the ET. Likewise D and B channel information destined for the ISDN subscriber are assembled within the ET in the prescribed ISDN signal format for transmission via the ISDN subscriber's line.
One of the problems in adapting existing digital circuit switching systems to the ET function in ISDN service is that of handling the D channel signalling and supervision information. In addition, packet switch destined data must be so identified and thereafter forwarded to an associated packet network. Signalling and supervision information destined for the controller in the digital circuit switch must be collected and translated into the appropriate format. Likewise signalling and supervision information destined for the ISDN subscriber must be translated into the ISDN protocol. Traditionally, the corresponding functions for analog telephones connected to a digital circuit switch have been performed in line interface circuits, there being one for each telephone line, for example as described by Harold Harris in the previously mentioned publication in TELESIS.
One of the problems in handling a D channel is the variable rate at which D channel information may occur. This information may have to be handled at the full sixteen kilobits per second rate in both transmit and receive directions. However, in contrast there may be no D channel information at all for significant periods of time. This variable rate dictates that either very fast signal processing apparatus be provided or alternately that a very large buffer memory be provided, on a per ISDN telephone line basis. One way or the other, the effective peak information capacity of the D channel handler is seldom ever used and frequently the D channel handler is inactive for extended periods of time. Provision for ISDN subscriber signalling and supervision in the line circuit, as suggested by tradition, promises to be expensive.